Power generating boilers using coal or petroleum coke as fuel have been heavily used. Reasons for the heavy use are low fuel costs, easy adjustment of electricity generated and so on, and therefore, developing countries such as China as well as Japan depend on coal and petroleum coke for most of electricity generated. However, coal and petroleum coke have a major disadvantage of discharge of a large amount of carbon dioxide.
To the world, Japan made a public commitment to reduce the amount of discharged carbon dioxide in the year 1990 by 25% until the year 2020. This commitment shows an extremely difficult numerical value to achieve, and the public and the Industry must fulfill their large obligations. However, because of having made the commitment, Japan must work toward the aim. Therefore, it is very important to reduce the amount of carbon dioxide discharged from coal and petroleum coke, which are used in the power generating boilers.
Since the use of coal and petroleum coke as fuels for power generation leads to discharge of a large amount of carbon dioxide, these fuels are regarded as sources of the all evil in terms of discharging of carbon dioxide. However, it is impossible for resource-poor Japan to immediately stop coal among all fossil fuels. At least until nuclear power generation and clean alternative energy are prepared, the use of coal cannot be stopped because of its economic efficiency, its convenience, rich reserve and difficulty in depletion.
Therefore, a future technically important object is to reduce the amount of carbon dioxide discharged from these fossil fuels as much as possible, and development of a new technology to attain this object is an essential theme. In this connection, pulverization in a grinding stage of coal and petroleum coke that are supplied to the boiler and reduction of the amount of generated carbon dioxide by the pulverization should be considered. Although the reduction effect achieved by one grinding mill is insignificant, the mills used all over the world is too numerous to count, which result in drastic reduction of the amount of discharged carbon dioxide. Advanced countries, in particular, Japan as a technology-oriented nation have the mission and obligation to take the initiative in working on the pulverization in the grinding mill.
The present inventors have noted this matter early on, worked on the pulverization in the grinding mill, and achieved great results. A typical technology is an improvement in the shape of a grinding surface of a roller, which is described in Patent Documents 1 and 2, in particular, development of a slit roller. In the slit roller, slit grooves extending in a center line direction (direction perpendicular to a roller circumferential direction) are formed in an outer circumferential surface as the grinding surface of the grinding roller at regular intervals in the circumferential direction. Thereby, as compared to the existing vertical roller mills, the biting property of ground matters and the pulverization rate are improved.
That is, in the case of a thermal power plant, at present, the ground coal grains passing through a 200 mesh screen are 75% on average. However, by further reducing the ground grain size so as to collect a larger amount of fine powder passing through the 200 mesh screen with over 75%, as compared with conventional mills, the combustion efficiency of the boiler is improved, enabling complete combustion and contributing a decrease in the amount of discharged carbon dioxide.
In producing pig iron in a blast furnace in a steelmaking plant, a large amount of coke reducing gas is generated and used to reduce and melt iron ore. Since coke is produced from expensive binding coal and is so expensive, in order to reduce the amount of used coke, inexpensive powdered coal is blown from tuyere of the blast furnace to decrease the amount of consumed coke, thereby cutting pig iron manufacturing costs.
The slit roller developed by the present inventors has been widely adopted in blast furnace powdered coal blowing equipment, which greatly contributes to cost reduction. It is said that the cost reduction effect in a certain steelmaking plant achieves as much as 600 million to 700 million yen annually. Since the amount of produced powder of 200 meshes or less is larger than that of conventional mills by about 20% or higher, the combustion efficiency of the blast furnace is improved, which contributes further reduction of the amount of consumed coke. In other words, the reduction of the amount of consumed coke leads to reduction of carbon dioxide occurring at production of coke, thereby largely contributing reduction of discharged carbon dioxide.
The vertical roller mill has been heavily used as a coal grinder in the power generating boiler. The vertical roller mill is configured of one horizontally-rotating driving table and a plurality of grinding rollers arranged on the driving table so as to surround the rotational center line, and coal supplied from the center of the mill to the center of the table is carried outward by a centrifugal force and pinched between the rollers and the table, thereby sequentially grinding coal. The ground coal is carried upward by carrying air, classified by a classifier. Out of the coal, coal of required grain size is captured and transferred to a subsequent stage, and coal of larger grain size is returned into the mill again.
The vertical roller mill for coal grinding is broadly classified into a Loesche type in which the shape of the grinding roller is truncated cone and an annular grinding part on an upper surface of the rotating table is a horizontal surface, and a tire type in which an outer circumferential surface of the grinding roller is curved in a plane vertical to the rotating direction so as to protrude toward the outer circumference, and an annular groove having an arcuate cross section, which is engaged with the outer circumferential surface of the grinding roller is formed on the upper surface of the rotating table. The tire-type grinding roller is further classified into a convex tire having a ratio of a maximum diameter D to radius of curvature R of a surface vertical to the rotating direction of the tire grinding surface of 4.3 or higher, and a flat tire having the ratio less than 4.3. According to the present inventors' research of D/R of the commercially available tire-type rollers, an average D/R of the convex tire is in the range of 4.5 to 5.0, and an average D/R of the flat tire is in the range of 3.8 to 4.1. Thus, D/R of 4.3 is reasonable as a diverging point of both D/R.
The present inventors have researched a screw roller in addition to the slit roller. The screw roller is a roller in which a plurality of screw grooves (spiral grooves) inclined relative to the roller circumferential direction are provided in parallel in the roller outer circumferential surface (Patent Documents 3, 4). The slit grooves in parallel to a roller shaft (vertical to the roller circumferential direction) are excellent in the biting property of a raw material, but is significantly high in the ability to scatter the material. On the contrary, circumferential grooves vertical to the roller shaft (roller circumferential direction) cannot obtain the good biting property of the ground raw material. By making the screw-like slit grooves so as to collect the ground raw material toward the center of the table, the amount of inserted ground raw material in a grinding space formed between the roller and the table increases. Thus, even in the case of the same roller clearance, a contact frictional force with the roller increases, thereby possibly preventing mill oscillation at a low-load operation and the like in the thermal power plant.
However, the long-term experience and experiment study of the present inventors demonstrate that the vertical grinding roller in which the slit grooves are formed on the entire grinding surface to improve the biting property and the grinding roller having the screw grooves that are excellent in the transfer property of the ground raw material have common problems.
That is, both in the roller with the slit grooves and the roller with the screw grooves, their added values cannot be completely exhibited for the ground raw material having a high hardness due to excessive wear, and the inventors have looked for its solution. If this problem is solved, the grinding roller with the slit grooves and the grinding roller with the screw grooves can realize a perfect vertical mill roller capable of sufficiently proving the merit of the grinding property for every grinding materials, that is, all of materials having a high hardness, materials having a high water content and adhesive materials, except for ignitable materials.
Then, the present inventors got back to the basic, and decided to clarify true functions and effects of the existing grinding rollers and develop a fundamentally new grinding surface. For this reason, the present inventors first examined problems common to the roller with the slit grooves and the roller with the screw grooves. As a result, two following problems related to the roller circumferential direction and the roller shaft direction emerged.
The first problem relates to wear of the grinding surface of the grinding roller in the roller circumferential direction (rotating direction). Details will be described below. When a hard material is ground, the slit grooves are disadvantageously prone to early wear. That is, conventionally, the slit grooves are formed in the entire roller grinding surface. In such a grinding roller, when a soft material is ground, wear of soft ribs constituting the slit grooves gradually develop to form the slit grooves, and wear-resistant hardened metal existing between the soft ribs appears in the shape of a gear. However, since the ground raw material is soft, the edge of the appeared hardened metal is not subjected to wear and holds to be almost vertical, resulting in that the excellent biting property and wear resistance are kept for a long time, thereby maintaining the effects and life of the roller and achieving a satisfactory use result. In the case where the soft raw material is ground, even when the slit grooves or the screw groove are formed in the entire roller grinding surface, the effects can be sufficiently obtained.
For example, in the case of grinding of coal having HGI of 45 or higher and grinding of slag in the blast furnace, the productivity can be greatly improved and the life can be largely extended.
On the other hand, when a very hard ground raw material is ground, the soft ribs constituting the slit grooves early wear, wear-resistant metal in the shape of a gear appears in a short time, and corners of the wear-resistant metal efficiently grind the hard material to improve the grinding efficiency. However, due to the hard material, the sharp gear-like shape extremely wears and early changes to a mountain-like shape, the grinding efficiency gradually lowers. At the same time, replacement is required within a short time as a result of the extreme wear. The wear speed is much higher than that of the existing circumferential wound build-up welding roller.
For example, for the cement raw material grinding roller used in a cement factory, the production volume per unit time increases by about 20% or more, but the life becomes a half of the existing build-up welding roller or shorter. Further, in the case where highly hard silica stone and ceramics, non-weathered blast furnace slag, and low-quality coal containing much ash are ground, the wear speed extremely increases.
Based on the phenomenon, the present inventors determined that the life of the roller with the slit grooves and the roller with the screw grooves did not depend on only the wear resistance of the adopted wear-resistant metal, and largely depended on the shape of the grinding surface. As an example, numeral analysis demonstrates that the pressure applied on the gear-shaped edge of the roller with the slit grooves by the wear-resistant hardened metal is about three times as much as the pressure applied on the circumferential wound build-up welded smooth grinding surface of the tire-type roller by the same hardened metal.
Since wear is generally proportional to the power of the pressure applied to the wear surface, it is assumed that the edge is subjected to wear that is 2 to 4 times as much as the pressure than the smooth surface. Accordingly, the pressing need is to develop a new grinding surface capable of exhibiting efficient grinding of the slit grooves even when the hard ground raw material, and moreover, ensuring the same life as that of the smooth grinding surface even when metal having the same wear resistance.
The second problem relates to wear of the grinding surface of the grinding roller in the roller shaft direction. That is, when observing wear of the grinding roller, in the grinding surface of the trapezoidal roller in a stage where the grinding efficiency lowers and the roller should be exchanged, a deep wear groove occurs on the large-diameter side, and no wear occur on the small-diameter side. In the tire-type convex roller having a small curvature (D/R=5), like the trapezoidal maximum wear occurs mainly on the large diameter, and the tire-type flat roller having a large curvature (D/R=4), maximum wear occurs on the small-diameter side.
It can be determined that the grinding part generating maximum wear is a part that contributes to the grinding most in the entire roller grinding surface, and has a largest ground amount, in which pulverizing is mainly performed. Although the other grinding surface also grinds fine powder as a matter of course, since it does not wear so much, it is assumed that the surface is a transfer surface that acts to feed the ground raw material supplied to the center of the rotating table to the main grinding surface by a centrifugal force rather to perform pulverizing. The transfer grinding surface is a part that first bites the raw material and serves to crush the material having a large grain size. It is assumed that the grinding property of fine powder can be greatly improved by improving the raw material transfer property on the transfer grinding surface by any means. At development of the slit grooves, the present inventors focused on only the biting property, but they developed the screw grooves capable of effectively grinding the adhesive substances such as limestone without adhesion to the roller and then, found the importance of the raw material transfer property of the grinding surface.
Theoretically considering, the roller grinding surface includes two grinding surfaces including the main grinding surface where pulverizing is mainly performed and the transfer surface where the raw material is fed to the main grinding surface. By clarifying role sharing of the grinding surfaces, any kinds of raw material can be transferred to the main grinding surface stably and reliably. This enables design of the grinding surface capable of reducing wasted energy necessary for grinding and performing grinding more efficiently, and prevents wear of the main grinding surface. This could be recognized based on long-term experience and trial and error from past to present.
As described above, one of important roles of the grinding surface is the raw material transfer property. In fact, it turns out that the existing smooth surface roller does not perform the function. When a hard ground raw material or a moist ground raw material is ground, since the grinding surface is a smooth surface, the biting property and the transfer property are poor, and the roller slips, thereby generating a large oscillation in the grinder itself to make its operation difficult. As a result, the production volume of fine powder decreases. When excessive pressure is applied to the roller in order to suppress slip and oscillation of the roller, an axis current of the mill increases, generating a large power loss.